A process for producing anhydrosugar alcohols, both monoanhydrosugar alcohols and dianhydrosugar alcohols, by dehydration of the associated sugar alcohols or monoanhydrosugar alcohols using a catalyst and an organic solvent is described, wherein the organic solvent is recycled during the process, and wherein the resulting anhydrosugar alcohols are very pure.
This application claims priority from German utility application 198 34 778, filed Sep. 9, 1998. Further, the following application Ser. No. PCT/US99/00539, filed on even date herewith, contains related subject matter: PROCESS AND PRODUCTS OF PURIFICATION OF ANHYDROSUGAR ALCOHOLS. The subject matter of each of the above-mentioned applications is hereby incorporated by reference.
Anhydrosugar alcohols, whether they are monoanhydrosugar alcohols or dianhydrosugar alcohols, are known to be produced with the aid of various acid catalysts by dehydration of the associated sugar alcohols or monoanhydrosugar alcohols. Examples of these catalysts include sulfonated polystyrenes (H+ form) (German Patent DE 3 041 673 C2; Canadian Patent Disclosure CA 1 178 288 A1); and various mineral acids, such as HCl (U.S. Pat. No. 4,169,152; German Patent Disclosure DE 3 233 086 A1), H3PO4 (East German Patent Disclosure DD 1 32 266; Can. J. Chem., 52 (19) 3362-72 (1974)), HF (International Patent Disclosure WO 89/00162 A; Carbohydr. Res. 205 (1990) 191-202) and H2SO4 (German Patent Disclosures DE 3 521 809 A1 and DE 3 229 412 A1).
These processes are often performed in the presence of a solvent. As solvents, water (CA 1 178 288 A1; European Patent Disclosure EP 0 052 295 B1) and organic solvents such as toluene or xylene (Przem. Chem. 48 (11) 665-8 (1969)) are known.
Batch processes for the preparation of dianhydrosugar alcohols by acid hydrolysis have been described in numerous patents and articles, for example, U.S. Pat. Nos. 3,454,603; 4,564,692; and 4,506,086; Canadian Patent 1178288; and articles J. Am. Chem. Soc., 68(5) pp. 939-941 (1946); J. Chem. Soc., p. 433-436 (1947); Przem. Chem. 48(11) pp. 665-668 (1969); and Pr. Nauk. Inst. Technol. Org. Tworzyw Sztucznych Politech. Wroclaw. No 3., p. 3-14 (1971).
In particular, a batch process for the formation of the dianhydrosugar alcohol isosorbide has been described in the literature as a two step process involving intramolecular dehydration of sorbitol to sorbitan (1,4-monoanhydrosorbitol), and further reaction of sorbitan to isosorbide (1,4:3,6-dianhydrosorbitol) in an acid catalyzed reaction, or dehydration, and cyclization. In this process, an aqueous solution of sorbitol is charged to a batch reactor. The temperature is increased to 130-135xc2x0 C. under vacuum (35 mm Hg) to remove the water. When the sorbitol melt is free of water, a catalyst, usually sulfuric acid, is added and the temperature and vacuum levels are maintained. The operable temperature range of the reaction is very narrow. Higher temperatures lead to decomposition and charring of the end product, while lower temperatures inhibit the reaction rate due to difficulties in removal of the water of reaction. This reaction produces isosorbide and a higher molecular weight byproduct. The byproduct is presumably produced by water elimination between two or more sorbitol molecules, but its exact nature is not clearly defined. See starch/stxc3xa4rke (1986), 38(c), 26-30 and Roland Beck, Pharm. Mfg Inc. (1996), 97-100.
As described above, the known processes for the production of anhydrosugar alcohols are discontinuous batch processes producing a high molecular weight byproduct. There is no known teaching of performing the process continuously, or of recycling the solvent for use during the process. However, a continuous process for the production of anhydrosugar alcohols is desirable to facilitate large scale, economical production of both mono- and dianhydrosugar alcohols.
There is also an absence of teachings in the art regarding purification of the produced anhydrosugar alcohol in order to achieve a level of purity acceptable for use in such end products as polymers. Polymers require a high degree of purity in the starting materials in order to achieve clarity in the end product. Impurities in the starting materials manifest as discolorations in the polymeric product, usually of a yellow to brown color. This coloration interferes with the use of the polymer for production of certain articles of manufacture, such as optical disks, fibers, films, sheets and containers, all of which require a high degree of clarity.
Crude anhydrosugar alcohols contain contaminants. For example, crude isosorbide contains the degradation products of sorbitol, sorbitan and isosorbide, which cause the crude isosorbide to have a light yellow to brownish color. Therefore, additional process steps for purifying the product are necessary before use.
Commercially available anhydrosugar alcohols are also of unacceptable purity for the production of polymers. For example, commercially available isosorbide, though purified and appearing white in its crystalline form, turns yellow or brown upon annealing to temperatures xe2x89xa7250xc2x0 C., which is lower than the temperature required for the formation of polymers, indicating that a resultant polymer would likely be discolored. Thus, a means of making a purer product than what is commercially available or available through known manufacturing processes is desired.
Processes known for use in purification of anhydrosugar alcohols include distillation, which can occur with or without the addition of boron compounds, for example in the form of boric acid (U.S. Pat. No. 3,160,641) or sodium borohydride, use of an anion exchange resin (U.S. Pat. No. 3,160,641), and recrystallization from organic solvents such as methyl ethyl ketone or ethyl acetate (U.S. Pat. No. 3,454,603).
The present disclosure relates to a process for producing very pure anhydrosugar alcohols which is economical and produces good yields, in which the formation of byproducts and contaminants is minimized, and which can be performed on a large industrial scale continuously without interruption for a relatively long time.
A preferred embodiment provides for a process of producing anhydrosugar alcohols wherein the process includes the steps of introducing at least one sugar alcohol or monoanhydrosugar alcohol into a reaction vessel; dehydrating the sugar alcohol or monoanhydrosugar alcohol in the presence of an acid catalyst and an organic solvent to form a reaction product which is at least partly soluble in the organic solvent; removing water from the reaction vessel; removing organic solvent comprising the dissolved reaction product from the reaction vessel; separating the reaction product from the removed organic solvent; and recycling the organic solvent into the reaction vessel.
Another preferred embodiment further includes purifying the separated reaction product by distillation, recrystallization, or a combination thereof.
Yet another preferred embodiment provides for a process of producing anhydrosugar alcohols wherein the process includes the steps of introducing at least one sugar alcohol or monoanhydrosugar alcohol into a reaction vessel; dehydrating the sugar alcohol or monoanhydrosugar alcohol in the presence of an acid catalyst and a solvent to form a reaction product which is at least partly soluble in the solvent; removing water from the reaction vessel; removing solvent comprising the dissolved reaction product from the reaction vessel; separating the reaction product from the removed solvent; and recycling the solvent into the reaction vessel, wherein the steps of introducing in the starting materials, removing water, removing solvent and recycling the solvent occur simultaneously.
In preferred embodiments, it is desirable that the solvent be an organic solvent, the acid catalyst be a soluble acid or an acid anion exchange resin, and the reaction product be separated from the solvent by means of extraction. However, other materials and methods are contemplated for use, as disclosed herein.